Detecting the first time I open/append to a file

Bruno Desthuilliers bdesth.quelquechose at
Tue Sep 23 19:17:43 CEST 2008

Sean Davis a écrit :
> On Sep 23, 2:02 pm, tkp... at wrote:
>> I have a simulation that runs many times with different parameters,
>> and I want to aggregate the output into a  single file with one rub: I
>> want a header to be written only the first time. My program looks a
>> bit like this:
>> def main():
>>     for param in range(10):
>>         simulate(param)
>> def simulate(parameter):
>>     'Lots of code followed by:
>>     with open(summaryFn, 'ab') as f:
>>         writer = csv.writer(f)
>>         writer.writerow(header)
>>         writer.writerow(Sigma)
>> If I can sense that the file is being created in the first iteration,
>> I can then use an if statement to decide whether or not I need to
>> write the header. Question: how can I tell if the file is being
>> created or if this its the first iteration? It's unrealistic to test
>> the value of the parameter as in the real problem, there are many
>> nested loops in main, and the bounds on the loop indices may change.
> You could use os.path.exists() to check if the file is there.
> However, the file could have been left over from a previous execution,
> etc.  What might make sense is to open the file only once, store the
> file handle, and write to that throughout the execution.

s/file handle/file object/

This being said, I can only second Sean's advice. And anyway, computing 
something and writing the result(s) of this computation to the file are 
orthogonal and mostly unrelated responsabilities, so they shouldn't be 
handled by the same function.

Here's a possible reorganisation of your code:

def simulate(parameter):
     # lots of code followed by
      return sigma

def main():
     # presumably some other code here
     with open(summaryFn, 'ab') as f:
         writer = csv.writer(f)
         for param in range(10):

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